Recognition and repair of the CC-1065-(N3-adenine)-DNA adduct by the UVRABC nucleases

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Abstract
The recognition and repair of the helix-stabilizing and relatively nondistortive CC-1065-(N3-adenine)-DNA adduct by UVRABC nuclease has been investigated both in vivo with .vphi.X174 RFI DNA by a transfection assay and in vitro by a site-directed adduct in a 117 base pair fragment from M13mp1. CC-1065 is a potent antitumor antibiotic produced by Streptomyces zelensis which binds within the minor groove of DNA through N3 of adenine. In contrast to the helix-destabilizing and distortive modifications of DNA caused by ultraviolet light or N-acetoxy-2-(acetylamino) fluorene, CC-1065 increases the melting point of DNA and decreases the S1 nuclease activity. Using a viral DNA-Escherichi coli transfection system, we have found that the uvrA, uvrB, and uvrC genes, which code for the major excision repair proteins for UV- and NAAAF-induced DNA damage, are also involved in the repair of CC-1065-DNA adducts. In contrast, the uvrD gene product, which has been found to be involved in the repair of UV damage, has no effect in repairing CC-1065-DNA adducts. Purified UVRA, and UVRC proteins must work in concert to incise the drug-modified .vphi.X174 RFI DNA. Using a site-directed and multiple CC-1065 modified (MspI-BstNI) 117 base pair fragment from M13mp1, we have found that UVRABC nuclease incises at the eighth phosphodiester bond on the 5'' side of the CC-1065-DNA adduct on the drug-modified strand. The enzymes do not cut the noncovalently modified strand. At low drug binding ratios. of the four CC-1065 binding sites indentified in the (MspI-BstNI) 117 base pair fragment, GATTA*, GGAAA*, GATAA*, and TTTTA* (* indicates the covalently modified adenine), only the adduct at the high-affinity binding site, GATTA*, is incised by the UVRABC nucleases. No difference in the effect of CC-1065 on local DNA structures, as determined by the DNase I cleavage pattern, was evident among these sites. At high drug binding ratios, a fifth drug binding site, AGCTA*, is identified. At this concentration UVRABC nucleases are unable to incise any of these five CC-1065-DNA adducts. The DNA sequence and/or helix-stabilizing effect of multiple adducts may determine the recognition and/or incision of the drug-DNA adduct by UVRABC nuclease. These results are discussed in relation to the structure of the CC-1065-DNA adduct and the effect of drug binding on local DNA structure.

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